How frequent is double pathology in Rasmussen encephalitis?

Rasmussen encephalitis (RE) is an uncommon, medically refractory cause of seizures that usually presents in childhood or adolescence resulting in unilateral hemispheric atrophy in most cases. The purported immune-mediated cause of the disease is supported by the characteristic histopathologic features of diffuse perivascular and intraparenchymal T-cell-predominant infiltrates, microglial activation with microglial nodules, and neuronophagia. A small number of reports have emerged, however, suggesting that double pathology (such as focal cortical dysplasia (FCD) or hippocampal sclerosis) may be present. We asked how often double pathology could be demonstrated in RE. A 15-year retrospective review of all RE cases seen at our pediatric tertiary care hospital yielded 11 patients, many of which had some degree of double pathology. The most diagnostically incontrovertible examples showed leptomeningeal neuronal heterotopia (n = 1) or leptomeningeal melanocytic nevus (n = 1). Another coexistent feature was neuronal gigantism unassociated with hemimegaloencephaly, particularly prominent in layer II (n = 2). Three additional cases showed dysmorphic neurons in the hippocampus (n = 2) and dentate granule cell layer dispersion/bilamination (n = 1). Finally, 2 cases had exaggerated radial arrangement of neurons FCD type Ia-like changes) (n = 2), a condition known to have interobserver discordance. In summary, 7/11 of our RE cases showed prominent additional pathologies, 6 of which demonstrated disproportionately more severe inflammation, neuronophagia, or microglial activation in regions of double pathology. Our collection of cases shows that a majority of RE cases show double pathology. Although some of these cases can be reactive, the presence of maldevelopment in a subset raises the possibility of a causal relationship with RE.